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This is a kind of follow-up question to "What orbital period would produce one New Moon (and one Full Moon) each year?"

Given the six-month orbital period that is needed to produce one New Moon and one Full Moon each year, what would such a satellite look like as it made its progress through the sky (assuming we are in a system that is as close to our current situation as possible)?

In other words, I am wondering things like:

  • How long would it take the moon to cross the sky?
  • What kind of moonrise and moonset times would we expect, and how would they vary with the seasons (if at all)?
  • What would be the effect on tides?
  • What other observable effects (observable by mere mortals going about their business, that is, rather than astronomers or astrophysicists) would result from a moon with a six-month orbital period?

Grateful for any insights here.

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A moon with a sixth month period would be about 1.3 million km from Earth. That puts it close to the edge of the Earth's Hill sphere, and probably isn't stable in the long term. So the biggest thing you might notice is "goodbye moon" (and having the moon in an Earth crossing orbit wouldn't be pleasant in the longer term, a collision would melt the crust)

Your questions:

  1. One day. The moon's motion is mostly due to the rotation of the Earth.
  2. The moon would be very distant and small but moonrise would happen just as it does now.
  3. Tides would be much reduced.
  4. The cycle from new moon to full moon would take much longer. Nights at full moon would be much darker than they are now. The moon is less likely to be tidally locked, so might show different faces at different times.
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Below is a five-year simulation of fictional circular orbits of Venus, Earth, Mars and Jupiter-like objects and two Earth moons with a period of 0.5 years; one prograde and one retrograde.

Earth's orbit is the ecliptic, all planets have inclinations of 10 degrees, moons are slightly split at 19.5 and 20.5 for visibility.

We see the familiar switch back and forth between apparent prograde and apparent retrograde motion against the stars for the planets, but the moon just moves smoothly in its inclined orbit.

Basically a 1/2-year moon would behave just like the 1/13-year moon we have now except do it more slowly and in apparent synchrony with the Sun.

  • For the prograde moon we'd have 1 full moons and 1 new moons per year.
  • For the retrograde moon we'd have 3 full moons and 3 new moons per year.

There would be nothing otherwise remarkable or interesting except for the annual or tri-annual nature of its phasing with respect to the seasons.

random simulation

Python script: https://pastebin.com/ZLTWr9g2

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